OUR SECOND DAY ON THE river, after a few mugs of cowboy coffee, we take a walk along the water. We study its current, test its temperature, survey the far bank with binoculars. And then we pull on our wetsuits, which Ruskey thinks we may need to avoid hypothermia if our swim goes longer than anticipated.

I say "we" because over last night's campfire I succeeded in finding two recruits to swim with me—two more loose screws from within our ranks.

Tom Roehm is a prominent Memphis environmentalist and an engineer by training who often canoes the river and its tributaries when he's not designing spinal implants for surgeons. "Are you kidding me?" said Roehm, a big, bearish, self-described "aquacentric" guy who used to compete as a distance swimmer. "I wouldn't miss this for the world. It's something I've wanted to do for 20 years."

Alan Spearman, a filmmaker and photographer with the local Commercial Appeal, recently made a documentary about a modern-day Huck Finn he met on the river. Spearman has, by one vessel or another, traveled every mile of the lower Mississippi, from Cairo to Venice, Louisiana. "I've swum in it before," Spearman said, "but never across it."

Ruskey has selected a passage of the river rather ominously called the Devil's Racecourse, so named (on old maps and even in Twain's Life on the Mississippi) because this stretch was once infamous for its steamboat-wrecking snags. Ruskey will paddle out in the channel with a few others in his 27-foot voyageur-style canoe, Ladybug, a noble vessel he hand-carved out of Louisiana bald cypress years ago. From out there, Ruskey will keep an eye on barge traffic while monitoring the radio for river-pilot chatter. If something goes wrong, he'll be close by.

"I guess it's showtime," I say.

"Here goes nothin'," adds Spearman.

I zip up my wetsuit and join Roehm and Spearman at the river's edge. I strap on a pair of ridiculous-looking split-fin flippers, which an open-water-swimmer friend of mine recommended. Then, together, we wade out into the mud.

"Good luck, Flipper!" hollers my college buddy Howard Stovall, a Memphis entertainment producer who grew up on the aforementioned Stovall Plantation—Muddy Waters's old stomping grounds. "You guys are insane. Man, you couldn't pay me to get in there!"

"And a fine 'fuck you' to you, too," I say.

I settle into the water, which is bracingly cold. I feel my heart pound, my skin tingle, my nerves race. I look across the channel to the Tennessee bank, clothed in a vegetational haze. Down here at river height, it suddenly seems a whole lot farther away than it did when I was standing up.

"OK, let's do this," Roehm says, and then we start swimming, following his lead.

For the first 30 yards or so, we swim through slackwater that's very easy going. Then we cross a distinct line of demarcation and hit the main channel, and suddenly we're fired downstream as if out of a cannon. It's impossible to fight this current even for a second. Any destination I might aim for on the far bank is now meaningless. I just have to let the flow carry me along and try to angle off it ever so slightly.

Lucky for us, Ruskey has shrewdly picked a crossing where the main channel shifts from the Arkansas side to the Tennessee side in a direct and pronounced swirl—which is to say, the current is moving in our favor. It's unnerving, at first, to be swept along by something so powerful, but after surrendering to it, I feel an intense exhilaration, as though I'm on a midway ride.

Soon we're scattered by the flow, and each of us swims alone, finding his own rhythm. The water surface is ripply and agitated now, slapping with crosscurrents, surging with boils. I can feel the river pressing in on all sides, grappling with me, trying to decide how best to deal with the impertinence of my presence. I recall something Ruskey told me several days ago. On fair days in the Grand Canyon, he said, you might find the Colorado River registering 10,000 cubic feet of flow per second. Here, the Mississippi is likely flowing at about 750,000.

Out in the middle of these 75 Colorados, I lose all sense of the current's velocity. At times, I think I'm not moving at all, but then I look back at the bank and see that, on the contrary, I'm hauling ass—effortlessly sliding down the nation's gullet.

"Sharpen your angle and pick up the pace!" Ruskey yells from the Ladybug. "There's a bad eddy down there on the Tennessee side—you'll want to make shore well before then."

"OK," I yell back. "But can you take these useless pieces of shit?" When Ruskey paddles by, I chuck my flippers into the canoe.

I dig in a little deeper—barefoot now and fully exposed to any lurking alligator gars. The river tastes like all rivers do: slightly metallic, alive with nutrients, a faint and not unpleasant hint of algae and fish. I don't know if dioxin has a flavor, but my taste buds aren't picking up anything funny— no tannic notes of Monsanto, no satiny finish of Dow.

What's unusual, though, is the grit. I've never swum in water this clouded with sediment, all that northern dirt flowing south. It is, of course, just that—good, clean dirt—but it works into my eyes, coats my tongue and nostrils, and crunches in between my clenched molars. In the old days, river pilots used to pride themselves on drinking a stout glass of this granular stuff every day, for good health. Nature's Metamucil!

Underwater, the sound is like a thousand bowls of Rice Krispies all popping at once. This, I conclude, must be the sound of untold tons of sediment tumbling on the river bottom, a great churning cloud somewhere below me.

It's 15 minutes into the swim, and I'm making good progress now. My home state—or, more specifically, some uninhabited place in Tennessee marked on Army Corps maps as Cedar Point—is drawing close. For me, this is a good, brisk workout, but if there's a feat in swimming across the Mississippi River, it's a feat more psychic than physical, more conceptual than aerobic. Any half-decent swimmer can do it.

Still, I hear myself chuckling. I can't believe I'm out here, doing this most exotic thing, which is also, given my background, the most obvious thing. It's as if I were some guy from Pamplona deciding, perhaps a little late in life, to go ahead and run with those demented bulls. I'm swimming across the Mississippi River! And I'm feeling strangely at home, as though I'm meant to be here, as though it belongs to me and I to it. There's joy in facing a natal fear—and in learning that, for today, at least, the river's not going to up and kill me.

I crawl toward the thickety bank, where stands of willow and cypress are choked in muscadine vine. Then, with my left hand, I touch the Great State of Tennessee. Total time from shore to shore: 27 minutes, and we've drifted nearly a mile downstream in the crossing. Roehm and Spearman have already made it in, a few minutes ahead of me. Soon River Jesus paddles by and, one by one, we pull ourselves into the Ladybug.

I look back toward Arkansas and savor my accomplishment. I'm pleasantly exhausted, coughing up a little river water and tugging on my neck for signs of an incipient goiter.

"Well, awwwrahhhht," Ruskey says. "Now the river is within you."

"It is, John—literally," I say, crunching on sand. "And I thanketh you."